Extrusion of PVDC

ABSTRACT

Adding a small amount [trace] of finely ground polyethylene to pvdc prior to extrusion, protects the pvdc from decomposing to hydrogen and carbon. This enables longer extruder run times, lower waste, and a higher quality product.

Polyvinylidene chloride (pvdc) is very difficult to extrude. Itdecomposes into hydrogen chloride and elemental carbon. Thedecomposition is catalyzed by contact with metal. During extrusion it isin contact with the extruder screw, the extruder barrel and the die. Itis common practice to start with a clean extruder and die. Polyethyleneis extruded first to coat thoroughly the metal parts of the extruder anddie. Extrusion of pvdc follows the polyethylene. The pvdc soon scoursthe polyethylene coating and gains metal contact. Decomposition of thepvdc starts with metal contact. A steadily increasing number of blackspecks of carbon occur in the product until the extruder is stopped forcleaning. This, of course, is an inefficient process.

This invention encapsulates the pvdc in a thin coating of polyethyleneto prevent the pvdc from metal contact. This produces a better product,free of black specks. The extruder is able to run for weeks or monthswithout having to be stopped for cleaning. The result is lower waste,higher production and a lower cost of production.

To encapsulate the pvdc finely ground polyethylene is intimately mixedwith the pvdc before extrusion. During extrusion the polyethylene meltsbefore the pvdv. Polyethylene is insoluble in pvdc. The small dropletsof polyethylene are squeezed out of the pvdc towards the extruder wallsor screw. A small amount (trace) of polyethylene encapsulates all of thepvdc. This prevents contact between the pvdc and the metal. Thisprevents metal catalyzed decomposion of the pvdc

The time that extruders are able to run is extended from days to months.The extruder barrel and screw are not coated with carbon black from thedecomposing pvdc. Instead they are coated with a very thin layer ofpolyethylene. This results in a tremendous decrease in waste. Shutdownsand startups create waste.

The quality of the product is improved. Black specks of carbon withinthe product lower the product quality.

In summary this invention improves the quality of the product. Itreduces waste. It increases the run time or the output of the individualextruder.

SUMMARY

Adding a small amount [trace] of finely ground polyethylene to pvdcprior to extrusion protects the pvdc from decomposing to hydrogenchloride and carbon. This enables longer extruder run times, lower wasteand a higher quality product.

1. The time that pvdc extruders are able to run without being stoppedfor cleaning is extended from days to weeks or months.
 2. The amount ofwaste generated is reduced.
 3. The quality of the product is improved bythe decrease in the black specks of carbon in the product.